The present invention relates to a valve mechanism of a multi-cylinder internal combustion engine, particularly such a valve mechanism in which a pair of air-intake valves provided on a cylinder can be closed together to make the cylinder pause and lift, of each air-intake valve can be changed independently in accordance with engine operation regions.
Hitherto, a valve mechanism of a multi-cylinder internal combustion engine in which a pair of air-intake valves provided on a cylinder can be closed together to make the cylinder pause and lift of each air-intake valve can be changed independently in accordance with engine operation regions is known (see Japanese Laid-Open Patent Publication No. 8-61031.
The above-mentioned valve mechanism comprises first and second drive rocker arms which are operatively connected to the respective air-intake valves and contacted with circular pause sections of a cam shaft, a first free rocker arm contacted with a substantial pause cam enabling the air-intake valve to open slightly, a second free rocking arm contacted with a low-speed cam for making the air-intake valve open with a small lift, and a third free rocker arm contacted with a high-speed cam for making the air-intake valve open with a large lift.
The first and second drive rocker arms and the first, second and third free rocker arms are connected or disconnected with each other suitably by switchover means so that the operation mode of the air-intake valves can be switched over. Namely, both the air-intake valves are closed by the pause sections in an engine pause state, one of the air-intake valves is made to perform opening-closing motion by the low-speed cam while another air-intake valve is made to substantially pause by the substantial pause cam in an engine low-speed operation region to produce swirl within a combustion chamber for improvement of combustion, and both the air-intake valves are made to perform opening-closing motion by the high-speed cam in an engine high-speed operation region to improve engine output.
In a rocker arm shaft of the valve mechanism are formed two switching oil-pressure supply passages of circular cross sections for supplying oil-pressure to the connection switchover means.
In the above valve mechanism, the operation mode of the air-intake valves is shifted from a state that one air-intake valve is made to perform opening-closing motion by the low-speed cam as well as another air-intake valve is made to substantially pause by the substantial pause cam in an engine low-speed operation region, to a state that both the air-intake valves are made to perform opening-closing motion by the high-speed cam in an engine high-speed operation. Accordingly, in a part of the engine low-speed operation region near the high-speed region where only one air-intake valve opens with the small lift, sufficient engine output cannot be obtained.
Since a plurality of the switching oil-pressure supply passages, which are usually formed by mechanical work and of relatively small diameter, must be provided within the rocker arm shaft, the working takes much time. Further, since each of the two switching oil-pressure supply passages formed in the rocker arm shaft has a circular cross section, the inner space of the rocker arm shaft is not necessarily utilized efficiently. Therefore, in order to ensure a necessary passage area of the switching oil-pressure supply passage, sometimes the diameter of the rocker arm shaft must be enlarged and it obstructs miniaturization of the valve mechanism.
The present invention has been accomplished in order to overcome the above difficulties, and a subject of the invention is to improve the nature of combustion in the combustion chamber and output of the engine, by increasing the number of operation modes of the air-intake valves depending on engine operation regions, in a valve mechanism of a multi-cylinder internal combustion engine having a cylinder provided with a pair of air-intake valves. Another subject of the invention is cost reduction and miniaturization of the valve mechanism.